In typical time division multiple access/time division multiplexing (TDMA/TDM) cellular systems (e.g., IS-136 and Groupe Special Mobile (GSM) systems), a diversity antenna scheme is utilized. For example, a typical cellular site employs a sectorized coverage scheme. For each sector, one transmit and two receive antennas are utilized. The two receive antennas are, for example, placed on a site tower and are separated by a sufficient number of wavelengths to avoid being correlated for the purposes of Raleigh fading. On the reverse-link (i.e., from the subscriber to the cell site receivers), a signal from a subscriber is received by both receive antennas. Both receive antennas are coupled to diversity ports of a receiver before digitally sampling and combining. Since the two antennas are sufficiently separated for the purposes of diversity, the combined signals from the two antennas provide a relative degree of robustness to Raleigh fading.
In urban environments, the varied morphology defined by the profile of buildings and other various obstacles presents unique challenges. In particular, the quality of a traffic channel within a particular coverage area or sector of a cell site may vary significantly as a subscriber moves through the particular coverage area or sector. The signal strength may experience “shadow fading” when a subscriber moves behind a corner of a neighboring building for example. Accordingly, the system may attempt to respond by appreciably increasing the transmit power associated with that subscriber. The increased power tends to increase the channel quality for that subscriber, at least initially. However, the increase in power tends to interfere with other subscribers in the same coverage area. Accordingly, these other subscribers may experience a decrease in quality and/or respond with similar increases in transmit power levels, thereby decreasing overall quality.
Moreover, it shall be appreciated that typical cellular antenna configurations are not effective at reducing shadow fading. For example, the discrete antennas of the typical base station antenna configurations discussed above are usually disposed from each other by approximately three to four meters. Accordingly, the difference in signal path, although sufficient for typical Raleigh fading characteristics, is quite negligible, and, hence, cannot sufficiently mitigate shadow fading characteristics associated with a dense urban environment or similar topological characteristics. It shall be appreciated that typical antenna configurations are limited to three to four meters due to spacing constraints associated with tower configurations at cell cites. For example, many individuals who lease space for cellular antennas are concerned with the aesthetics associated with the cellular antennas and will not permit an unduly bulky antenna system apparatus to be visible. Accordingly, base stations have been developed to utilize this common antenna configuration. Such base station configurations have, therefore, tended to use the typical antenna configuration even when deployed in different environments, such as upon the face of a building in a dense urban environment, as the base station equipment does not require substantial modification for use and the antenna configuration provides a deployment that has gained at least a certain amount of acceptance by property owners.